If you are looking for the most absorbable form, it's a sublingual B12 supplement. Sublingual allows for bypassing the gut altogether, which is better for people who may have trouble absorbing the vitamin.

It has to be added (not all of them have it) - so be sure to check the label. It's a fine source if you make sure to consume it regularly. Sometimes an actual supplement is easier to remember to take than yeast flakes.

As far as I know, it's shelf stable. What you do want to watch is getting water or other cross contaminants in it. Always use a dry, clean spoon or shake it out.

A couple of months? wow! We use ours up pretty regularly - I buy it in the bulk bin so I buy what we are likely to use in about 6 weeks at a time. We also keep ours in the fridge just on general principles, though it doesn't necessarily need refrigeration. That way, on the occasions that we don't use it as quickly, I don't get concerned about it.